Deployment of wireless communication systems are increasing. Given crowded frequency bands and diverse requirements for multi-frequency communication, antenna structures able to perform in one or more bands or with switchable directionability characteristics are of great interest. One solution here is the use of reconfigurable antennas or other structures (e.g., reflective structures). Generally speaking, these are antennas or associated resonant structures which may have their frequency and/or their directional characteristics altered so as to perform in one or more frequency bands and/or with one or more directional beams.
Reconfigurable antenna structures have been used for some time, where elements of the structure are connected and disconnected by a switch. PIN diodes and GaAs field effect transistors (FETs) have been used to perform these switching operations. Such switching devices typically require a bias current and corresponding circuitry, making their use cumbersome. The advent of micro-electro-mechanical systems (MEMS) has allowed the creation of ultra-small switches. The introduction of MEMS switches has created new possibilities in the RF communications field.
For example, multiple ground planes behind a single radiating element may be switched in or out of the circuit using an array of MEMS switches. The MEMS switches can be constructed as bi-stable devices and are switched from one position to the other by the application of a DC voltage to an input terminal. Any DC voltage source may be used to activate the MEMS switches. Conventionally, the DC activation voltage is delivered to the switch by conductive material, such as copper wire or a copper run on a printed wiring board.
In high frequency (e.g., microwave, millimeter wave) applications, however, the introduction of copper or other conductive materials into or near an RF structure may have an undesirable effect. For instance, added wires and conductors may scatter the RF fields around antenna elements, which distorts the antenna radiation patterns or affects the antenna impedance. In some applications, the switch control wires can be concealed by the antenna elements or their RF feeds, thereby minimizing the interference with the operation of the antenna. However, only a few antenna elements allow embedding of the control lines. To address this problem, strategies have been developed to use a photovoltaic cell to generate the DC switching voltage for the MEMS switch, as shown in the system 100 of FIG. 1.
Here, MEMS switch 102 is attached to a photovoltaic cell 104. An optional capacitor 106 may be utilized at the switch input. A laser beam 108 illuminates photovoltaic cell 104, causing the MEMS switch 102 to change states. A passive antenna element or other structure connected thereto is then switched in or out of the circuit. Laser light 108 is generally conducted to photovoltaic cell 104 by an optical fiber. Unfortunately, the running of optical fiber from a laser light source to the MEMS switch 102 is not practical for many applications. Moreover, if the switch 102 must be enclosed in an opaque material, then neither visible nor infrared (IR) light can be used to activate them effectively.
What is needed, therefore, is a MEMS switch that can be activated without adversely affecting antenna structure performance. In a more general sense, there is a need for a MEMS switch that can be activated transparently to the application it is supporting.